1. Field of the Invention
The present invention relates to a cooling technology for a battery pack including a plurality of battery modules connected in series or in parallel, particularly, to a cooling technology for secondary batteries used in hybrid electric vehicles (HEVs) or pure electric vehicles (PEVs).
2. Description of the Related Art
Secondary batteries are of various types: lead-acid batteries, nickel-cadmium (Ni—Cd) batteries, nickel-metal-hydride (Ni—MH) batteries, and lithium ion batteries. After being discharged, these batteries can be recharged by a predetermined electric current supplied from an external power source. Such characteristics allow them to be used in various kinds of equipment. For example, batteries have been used in vehicles to deliver power to the spark plug of an engine.
In recent years, a Ni—MH battery has been used largely as a main power source for driving electric motors in a pure electric vehicle (PEV) and a so-called hybrid electric vehicle (HEV), which includes an engine and electric motor. This is because a Ni—MH battery has high energy density, i.e., it can store the energy in a compact manner, and high power density. To deliver sufficient power to their electric motors, PEVs and HEVs employ battery packs; a battery pack is built by combining a plurality of cells into a battery module and connecting two or more battery modules in series or in parallel.
In a Ni—MH battery, which is formed by combining a plurality of battery modules and used in PEVs and HEVs, a large charge/discharge current flows repeatedly because of braking, acceleration, or the like of the vehicle during driving. This causes I2R losses due to the internal resistance of the Ni—MH battery, resulting in heat generation in the battery.
Compared with a lead-acid battery having a large weight, a Ni—MH battery provides high energy density, i.e., it can store the energy in a compact manner. Thus, a plurality of battery modules can be combined compactly. However, such a structure makes heat dissipation in the Ni—MH battery more difficult than that in the lead-acid battery.
To solve the above problems, the method in which a battery is cooled by forcing a coolant, such as air or the like, into the gaps between battery modules is known. In this case, cooling performance can be improved by narrowing the gaps between battery modules and increasing the flow velocity of the coolant.
For example, U.S. Pat. No. 5,879,831 proposes a method for determining the optimum dimensions of the gaps between battery modules in cooling design, focusing on the fact that making the gaps narrower than necessary increases the flow resistance and reduces the flow rate, which results in poor cooling performance.
However, the gap dimensions determined by this method are often reduced to such an extent that machining accuracy becomes more important. In that case, the variation in the gap dimensions causes non-uniform cooling in the battery pack.
Furthermore, the non-uniform cooling causes unbalanced battery capacity, and thus the available area of the battery is limited. In a worst case, this might lead to serious trouble, e.g., the vehicle breaks down on the road.